1. Field of the Invention
The present invention relates to a method and a device for selecting a connection destination for wireless communication from a plurality of base stations in a wireless terminal located within a service area of the plurality of base stations. In particular, the present invention relates to a method for selecting a base station so that hand-over can be performed smoothly in a mobile phone.
2. Description of the Prior Art
Recently, mobile communication using mobile phones or PHS phones has been widespread astonishingly. For the mobile communication, wireless terminals (mobile terminals) such as mobile phones, wireless base stations, base station controlling devices, exchanges and the like constitute a wireless communication system (a mobile communication system).
Along with widespread mobile communication, services using various types of wireless communication systems such as PDC, GSM, W-CDMA, CDMA-2000, W-LAN (IEEE802.11), WiMAX (IEEE802.16e) and the like have become available due to advancement of systems and user's taste. These services have different features directing high speed communication, or directing increase of moving speed, or directing increase of service areas or the like.
Under such a situation, hand-over between wireless communication systems, i.e., hand-over between different wireless communication systems is required, so making a specification of hand-over between W-CDMA and WiMAX is being under study in 3GPP (Third Generation Partnership Project).
On the other hand, in the home market of Japan, for example, there is a dual terminal that can support communication between PHS and PDC or between PDC and FDMA (W-CDMA) that is available in the market. However, this terminal is in the situation in which hand-over cannot be performed.
In addition, hand-over in the same system is decided based on reception quality in the terminal (wireless terminal). This is also followed for hand-over between different systems in many cases. For example, EP patent No. 1009183 proposes a structure in which all the different systems transmit common data (common channel) and decide quality of the common channel, and make connection to a system having high quality for performing hand-over.
In addition, U.S. patent application publication No. 2002/0082017 proposes a structure in which decision of reception quality to be an indicator for hand-over is performed in a period depending on a moving speed of a terminal. In other words, the decision period is increased if the moving speed is slow, while it is decreased if the moving speed is fast.
Furthermore, U.S. patent application publication No. 2005/0119001 discloses a structure in which a server that manages wireless communication systems by which a terminal can make communication is provided, and the terminal sends a request to the server when the terminal performs hand-over. In this case, the server informs the terminal of a communication destination having a shortest path, i.e., time for hand-over as a hand-over destination.
However, in recent communication systems, a common channel for dispatch information and individual channels for transmitting user information are provided with different systems having different transmission powers and different structures of antenna. Therefore, in EP patent No. 1009183, even if the common channel has high quality, it may have opposite effect in the case where the relationship is not quality of the individual channels.
The case of U.S. patent application publication No. 2002/0082017 can reduce unnecessary hand-over processes so that power consumption of the terminal can be reduced. However, if a radius of the cell is large and a distance from the base station is small, i.e., even if possibility of hand-over is very low, the decision period becomes short if the moving speed of the terminal is high, so power consumption cannot be reduced.
According to the method of U.S. patent application publication No. 2005/0119001, high speed of hand-over time can be achieved. However, cost of the system increases because installation of the server is necessary.
As described above, there are various advantages and problems in the conventional process for hand-over. In any cases, it is surely advantageous for the wireless terminal located within service areas of a plurality of base stations to select a base station having a highest quality when hand-over is performed or when it is activated, so as to improve stability of communication and to reduce unnecessary processes for hand-over.